The present invention relates to an imaging optical system, and particularly to a new and useful way of producing control information for the imaging optical system. The present invention is particularly useful in providing control information for an imaging optical system such as a lithographic imaging optical system.
In a typical lithographic imaging optical system, a spatially incoherent radiation (e.g. light) source is used to illuminate a mask or reticle, to produce an image that is projected and used to image the photoresist on a semiconductor wafer. The wafer is typically supported on a machine part known as a wafer stage that can be moved (adjusted) relative to the imaging lens. A controller drives actuators associated with the wafer stage to correctly position the wafer stage in the imaging optical system.
In an imaging optical system such as a lithographic imaging optical system, it is desirable to position the wafer properly in relation to the focal plane of the imaging lens system. One way of generating control information for positioning a wafer relative to the imaging lens system is shown in U.S. Pat. No. 5,268,744. A reflection beam is directed through the optical system to a first region of a wafer and then onto a predetermined plane to determine the focal plane of the optical system, and to establish a reference position on the predetermined plane that corresponds to the focal plane. Another reflection beam is directed to another region of the wafer and onto the predetermined plane, and generating control information for positioning the other region of the wafer on the basis of the location of the other beam with respect to the reference position.